SOURCE

<div class="input">
    <input placeholder="请输入" />
</div>

.input {
    position: relative;
    width: 200px;
}
.input>input {
    width: 100%;
    padding: 6px 8px;
    box-sizing: border-box;
}
.input::after {
    content: "[ ]";
    color: red;
    text-align: justify;
    text-align-last: justify;
    pointer-events: none;
    position: absolute;
    left: 2px;
    right: 2px;
    top: 3px;
    bottom: 0;
}